Secondary batteries, which are easily applicable according to production group and have electrical characteristics, such as a high energy density, have widely been used not only for portable devices but also for electric vehicles (EVs) or hybrid electric vehicles (HEVs), which are driven by an electric drive source, and energy storage systems. The secondary batteries not only have a primary advantage of remarkably reducing the use of fossil fuels, but also generate no by-products due to the use of energy. Thus, the secondary batteries have attracted attention as a new energy source for enhancing eco-friendliness and energy efficiency.
A battery pack applied to the EVs has a structure in which a plurality of cell assemblies including a plurality of unit cells are connected in series, in order to obtain high power. Also, each of the unit cells includes positive and negative electrode current collectors, a separator, an active material, and an electrolyte solution, and may be repeatedly charged and discharged by an electrochemical reaction between components.
In recent years, with the increasing necessity for a large-capacity structure that can also be used as an energy storage source, a demand for a multi-module battery pack in which a plurality of battery modules, each of which includes a plurality of secondary batteries connected in series and/or in parallel, is assembled has increased.
Since the multi-module battery pack is manufactured such that a plurality of secondary batteries are closely packed in a narrow space, it is important to easily discharge heat generated in each of the secondary batteries. Since a process of charging or discharging the secondary battery is enabled by the electrochemical reaction as described above, if heat of the battery module generated in the charging/discharging process is not effectively removed from the battery module, heat accumulation occurs. As a result, deterioration of the battery module may be promoted and, in some cases, ignition or explosion may occur.
Therefore, a high-output large-capacity battery module or a battery pack including the battery module necessarily requires a cooling device configured to cool battery cells embedded in the battery module or battery pack.
In general, since an amount of power that can be produced by one battery cell is not large, a commercially available battery module includes a stack in which a required number of battery cells are stacked. In order to appropriately maintain a temperature of the battery module by cooling heat generated during the production of electricity in a unit battery cell, cooling fins are inserted between the battery cells. The cooling fins, which have absorbed heat in each unit cell, transmit the heat to a cooling plate and, and the cooling plate is cooled by a heat sink.
However, a conventional battery module has a limitation in that heat generated in a battery cell cannot be smoothly discharged to the outside due to a low thermal conductivity of the battery module. For example, in the conventional battery module, due to an assembling tolerance between cooling fins and a cooling plate that are fastened to cartridges, the cooling fins do not completely contact the cooling plate and many portions are lifted, so a thermal contact resistance at a contact surface between the cooling fins and the cooling plate is considerably high. Particularly, referring to FIG. 1, when a battery cell 1 swells due to a rise in temperature, an expansion pressure of the battery cell 1 is transmitted to a cooling fin 2 so that contact performance between the cooling fin and a cooling plate may further deteriorate.